With continuously increasing of energy density requirements for a lithium-ion secondary battery by the user, the applied charging cut-off voltage of lithium cobalt oxide is increased from the early 4.2V to the current 4.35V or more, while the compacted density of that is increased from the early 3.80 g/cm3 to the current 4.15 g/cm3 or more through matching optimization of particle size. The deterioration problem of the cycling stability of lithium cobalt oxide under the application of high voltage is brought about by increasing the charging cut-off voltage to enhance the deintercalation degree of lithium ions for the layer structure of lithium cobalt oxide to increase the capacity. In view of this problem, corresponding solutions are opened by many literature and patents, and mainly can be summarized as the following categories:
(1) Stabilizing the structural stability of the O—Co—O layer by multiple cationic or anionic doping to suppress irreversible structural changes in the application of high voltage cycles. The anions involved mainly are F, S. The cations involved are mainly composed of divalent to tetravalent cations and contain many kinds of elements, such as Mg, Ca, Sr, Ba, B, Al, Ga, In, Y, Ce, Ti, Zr, V, Cr, Mo, W, Mn, Fe, Ni, Cu, Ag, Zn, and so on, covering almost all possible elemental ranges (U.S. Pat. No. 7,459,238B2, U.S. Pat. No. 7,026,068B2, U.S. Pat. No. 8,178,238B2, etc.). However, not all of the cationic elements are suitable to be doped elements of lithium cobalt oxide. For example, Cu, Fe, Zn are considered to be dangerous elements to affect the self-discharge and safety of lithium cobalt oxide battery, and there is a need to strictly control their content to ppm level; and in other elements which have been disclosed, only a few can be used to make lithium cobalt oxide crystal grow smoothly to 18-25 μm, in order to obtain a higher compacted density through matching of particle size. Although there is a certain effect on the cycling stability of lithium cobalt oxide under high voltage through doping of surplus few elements, its effect is limited. Too many doping amounts will affect the deintercalation/intercalation kinetic properties of lithium ions, which can not fully meet application requirements of lithium cobalt oxide in a voltage of 4.45V or more.
(2) The surface of lithium cobalt oxide is protected through being coated by multiple inactive elements. The common coating elements include Al, Mg, Ti, Zr, Y, Mo, etc. Usually, simple element coating has a greater impact on the deintercalation/intercalation dynamics, resulting in deterioration of capacity, magnification and low temperature performance, meanwhile, with reference to the cycling stability under high voltage, it also can not fully meet application requirements of lithium cobalt oxide in a voltage of 4.45 V or more by simple element coating.
(3) The coating layer has lithium ion deintercalation/intercalation activity. Typically, the inner core is doped or undoped lithium cobalt oxide, and the shell coating layer has a lithium ion deintercalation/intercalation activity. For example, in a Li—Ni—Mn—O coating patent by Sony Company and 5V material (such as LiCoPO4) coating patent by other companies, this method can improve the cycling stability of lithium cobalt oxide under high voltage, while overcoming a variety of issues brought by the simple bulk phase doping or inactive elements coating. Influences of the molar ratio of Li to active element (Ni+Mn) in the coating layer on the high voltage cycling stability is not mentioned in the Li—Ni—Mn—O coating patent (U.S. Pat. No. 7,906,239B2, U.S. Pat. No. 8,445,129B2, U.S. Pat. No. 8,748,042B2, U.S. Pat. No. 9,190,660B2) of Sony Company.